Non-volatile ferroelectric thin film device using an organic ambipolar semiconductor and method for processing such a device

ABSTRACT

A non-volatile ferroelectric memory device is proposed which comprises a combination of an organic ferroelectric polymer with an organic ambipolar semiconductor. The devices of the present invention are compatible with—and fully exploit the benefits of polymers, i.e. solution processing, low-cost, low temperature layer deposition and compatibility with flexible substrates.

The present invention relates to non-volatile ferroelectric memorydevices and methods of making the same, which may, for example, becompatible with polymer processing methods. More in particular, thepresent invention relates to non-volatile ferroelectric memory devicescomprising a combination of a ferroelectric polymer-insulating layerwith an organic ambipolar semiconductor.

Memory technologies can be broadly divided into two categories: volatileand non-volatile. Volatile memories, such as SRAM (Static Random AccessMemory) and DRAM (Dynamic Random Access Memory), lose their contentswhen power is removed while non-volatile memories, which are based onROM (Read Only Memory) technology, do not. DRAM, SRAM and othersemiconductor memories are widely used for the processing and high-speedstorage of information in computers and other devices. In recent yearsEEPROMs and Flash Memory have been introduced as non-volatile memoriesthat store data as electrical charges in floating-gate electrodes.Non-volatile memories (NVMs) are used in a wide variety of commercialand military electronic devices and equipment, such as e.g. hand-heldtelephones, radios and digital cameras. The market for these electronicdevices continues to demand devices with a lower voltage, lower powerconsumption and a decreased chip size. EEPROMs and Flash Memory,however, take long time to write data, and have limits on the number oftimes that data can be rewritten.

As a way to avoid the shortcomings of the types of memory describedabove, ferroelectric random access memories (FRAMs), which store data bythe electrical polarization of a ferroelectric film, were suggested.There are two types of memory operation principles for ferroelectricmemories. A first type is detecting the amount of stored electriccharges i.e. the differential between the polarization switching chargecurrent and the polarization non-switching charge current (FRAM). Asecond type is detecting the difference of the FET channel conductance(FET). This channel conductance is modified by the polarizationdirection of the ferroelectric film on the FET channel region.Ferroelectric non-volatile memories are attractive, since they haveunchallenged performance advantages over current technologies (EEPROM,flash), such as higher write endurance, lower write voltage,non-destructive reading and lower power consumption.

Ferroelectric materials are characterized by spontaneous polarization inthe absence of an electric field, that is reversible upon application ofan electric field lower than the breakdown field. Spontaneouspolarization in a ferroelectric material arises from anon-centrosymmetric arrangement of ions or polar molecules in its unitcell that produces an electric dipole moment.

When an alternating electric field is applied to a ferroelectricmaterial the polarization shows a hysteresis behavior with the appliedfield. In an initial stage, ferroelectric domains that are orientedfavorably with respect to the applied field direction grow at theexpense of other domains. This continues until total domain growth andreorientation have occurred. At this stage, the material has reached itssaturation polarization (P_(s)) If the electric field is then removed,some of the domains do not return to their random configurations andorientations. The polarization at this stage is called the remnantpolarization (P_(r)). The strength of the electric field required toreturn the polarization to zero is the coercive field (E_(c)).

A typical ferroelectric hysteresis loop is illustrated in FIG. 1,showing surface charge density D in function of applied electric fieldE. At zero applied field E=0, there are two states of polarization,±P_(r). Furthermore, these two states of polarization are equallystable. Either of these two states could be encoded as a “1” or “0” andsince no external field is required to maintain these states, the memorydevice is non-volatile. To switch the state of the device, a thresholdfield with an absolute value larger than E_(c) is required. In order toreduce the threshold field E_(c) for a given ferroelectric material, theferroelectric material needs to be processed in the form of thin films(preferably with a thickness less than 2 micron).

The ferroelectric film on the memory cell capacitor may be made ofinorganic materials such as: barium titanate (BaTiO₃), lead zirconatetitanate (PZT—Pb(Zr, Ti)O₃)), PLZT ((Pb,La)(Zr,Ti)O₃)) or SBT(SrBi₂Ta₂O₉), or of organic molecular materials such as: triglycinesulphate (TGS) or organic polymers and oligomers with polar groups suchas e.g. odd numbered nylons, polyvinylidene cyanide p(VCN) orpolyvinylidenefluoride (p(VDF). From the polymers known to date,especially a group of fluorine containing materials, to which p(VDF)having the chemical structure (CH₂—CF₂)_(n) belongs, is preferred due toadvantageous properties, such as: high remnant polarization andrelatively low coercive field in films obtained directly formspincoating. Especially materials with combinations of VDF (CH₂—CF₂),with TrFE (CHF—CF₂) and/or TFE (CF₂—CF₂) such as for example the randomcopolymers (CH₂—CF₂)_(n)—(CHF—CF₂)_(m) or (CH₂—CF₂)_(n)—(CF₂—CF₂)_(m)have excellent ferroelectric and film forming properties. It is furthernoted here that in general any material that has a crystalline phasewith a crystal structure belonging to an asymmetric space group couldpossess ferroelectric properties as long as the electrical breakdownfield is higher than the required switching field (related to coercivefield).

However, in case of ferroelectric liquid crystalline polymers forexample, which are being used for, for example, displays, the remnantpolarization P_(r) is generally low (˜5-10 mC/m²), being dependent on adipole moment from a large molecule. This may be too low for memoryapplications. In addition, operating conditions will be very temperaturesensitive due to the liquid crystal properties. For memory applicationone likes to have stable properties at temperatures in betweenapproximately −20 to 150 C. Therefore, in case of non-volatile memorycells, preferably the aforementioned non-liquid crystalline organicferroelectric materials are used as a ferroelectric layer.

In US 2003/0127676 a non-volatile memory device 10 is described,including a substrate 1, an active layer 2, a drain 3, a source 4, agate insulating layer 5 and a gate 6. The active layer 2 is formed of anorganic semiconductor in a contact region between the source 4 and thedrain 3. The gate-insulating layer 5 is formed of a ferroelectricmaterial and is deposited onto the active layer 2, and the gate 6 isformed on top of the gate-insulating layer 5. The device 10 of thisdocument is illustrated in FIG. 2. Since the non-volatile memory device10 includes a ferroelectric gate insulating layer 5 and an organicsemiconductor active layer 2, it is very flexible, light-weight,multi-programmable and can be easily manufactured.

However, devices 10 having a unipolar organic semiconductor active layer2, only function in accumulation or depletion. Those devices 10 do notfunction in inversion. For ferroelectric transistor applications thisimplies that only for one polarization direction of the ferroelectricaccumulation charge density compensates for the polarization. For theopposite polarization direction the semiconductor is depleted and hencecompensation charge density must be present as space charges, i.e. thesemiconductor must have sufficient background doping to be able tosustain this polarization induced charge density. However, in order tonot deteriorate the transfer characteristics of organic transistors, thesemiconductors used, by their nature, do not comprise doping.Nevertheless they generally comprise impurities introduced duringsynthesis or handling and are often unintentionally doped. Thisunintentional doping apparently is able to facilitate some switching andprovide some stabilization of the involved gate polarization state. Thisoperation principle is however undesirable from a transistor quality andtechnological point of view since unintentional doping is notcontrollable and in fact undesirable. In addition, unintentional dopingmainly consists of ionic species or polar small molecules (space charge)that can be transported through the gate dielectric either underinfluence of the electric fields employed during device operation orthat can move after writing a certain polarization state thereby causingimprint, fatigue or other degradation phenomena often encountered inferroelectric memory devices.

It is an object of the present invention to provide non-volatileferroelectric memory devices, which can be obtained by low-costprocessing and at low temperatures, which are compatible with flexiblesubstrates and which solve the charge stabilization problem of the priorart devices.

The above objective is accomplished by a method and device according tothe present invention.

The present invention provides a non-volatile memory device comprisingan organic ambipolar semiconductor layer and an organic ferroelectriclayer. The organic ambipolar semiconductor layer and the organicferroelectric layer are at least partially in contact with each other.

In one embodiment of the present invention, the device may comprise acontrol electrode, which may be formed in a first conductive layer. Thefirst conductive layer may for example be a metal (e.g. indium Tin Oxide(ITO), gold), or a conductive polymer layer (e.g. PEDOT/PSS). Thecontrol electrode may be separated from the organic ambipolarsemiconductor layer by the organic ferroelectric layer.

The device according to the present invention may furthermore comprise afirst and a second main electrode. The first and second main electrodemay be formed in a second conductive layer. The second conductive layermay for example be a metal (e.g. ITO, gold), or a conductive polymerlayer (e.g. PEDOT/PSS). The control electrode may be separated from theorganic ambipolar semiconductor layer by the organic ferroelectriclayer. The first and second main electrode may be separated from eachother by material of the organic ambipolar semiconductor layer and maybe separated from the control electrode by the organic ferroelectriclayer.

In one embodiment of the invention the organic ferroelectric layer maybe a ferroelectric fluorinated polymer or oligomer layer and may forexample comprise material selected from (CH₂—CF₂)_(n), (CHF—CF₂)_(n),(CF₂—CF₂)_(m) or combinations thereof to form (random) copolymers like:(CH₂—CF₂)_(n)—(CHF—CF₂)_(m) or (CH₂—CF₂)_(n)—(CF₂—CF₂)_(m).

The organic ambipolar semiconductor layer, used in the invention, mayfor example comprise a mixture of an n-type and a p-type semiconductormaterial, such as for example a mixture of [6,6]-phenyl-C61-butyricacidmethylester and poly[2-methoxy,5-(3,7)dimethyl-octyloxy]-p-phenylenevinylene.

In another embodiment of the present invention, the organic ambipolarsemiconductor layer may comprise a single organic material, such as forexample poly(3,9-di-tert-butylindeno[1,2-b]fluorene).

Furthermore, the organic ambipolar semiconductor layer may be a doublelayer-stack of p-type and n-type semiconductors, wherein two activesemiconductors may be used. The non-volatile memory device of thepresent invention may comprise a memory window, whereby the memorywindow may depend on the ratio of electron current and hole current. Inone embodiment, the ratio of electron current and hole current may beclose to 0 or may be close to 1. In that case, the memory window may belargest.

The present invention furthermore provides a method for processing anon-volatile memory device. The method comprises:

forming an organic ferroelectric layer and

forming an organic ambipolar semiconductor layer,

the organic ambipolar semiconductor layer and the organic ferroelectriclayer being at least partially in contact with each other. The organicferroelectric layer may for example be a ferroelectric fluorinatedpolymer or oligomer layer and may for example comprise material selectedfrom (CH₂—CF₂)_(n), (CHF—CF₂)_(n) (CF₂—CF₂)_(m) or combinations thereofto form (random) copolymers like: (CH₂—CF₂)_(n)—(CHF—CF₂)_(m) or(CH₂—CF₂)_(n)—(CF₂—CF₂)_(m). The organic ambipolar semiconductor layermay be a mixture of an n-type and a p-type semiconductor material andmay for example be a mixture of [6,6]-phenyl-C61-butyricacid methylesterand poly[2-methoxy,5-(3,7)-dimethyl-octyloxy]-p-phenylenevinylene. Inanother embodiment, the organic ambipolar semiconductor layer may be asingle organic material such as for examplepoly(3,9-di-tert-butylindeno[1,2-b]-fluorene). Furthermore, the organicambipolar semiconductor layer may be a double layer-stack of p-type andn-type semiconductors, wherein two active semiconductors may be used.

The method of the present invention may furthermore comprise forming acontrol electrode from a first conductive layer. The first conductivelayer may for example be a metal (e.g. ITO, gold), or a conductivepolymer layer (e.g. PEDOT/PSS).

In one embodiment of the invention, the method may furthermore compriseforming a first main electrode and a second main electrode from a secondconductive layer. The second conductive layer may for example be a metal(e.g. ITO, gold), or a conductive polymer layer (e.g. PEDOT/PSS). Thefirst and second main electrode may be separated from each other bymaterial of the organic ambipolar semiconductor and may be separatedfrom the control electrode by the organic ferroelectric layer.

The method of the invention may furthermore comprise patterning of theorganic ambipolar semiconductor layer.

Advantages of the device of the present inventions are that it can bemade by means of solution processing and hence low-cost processing canbe achieved. Another advantage of the present invention is that thedifferent layers, necessary to form the device, can be deposited at lowtemperature. A further advantage of this invention is the compatibilitywith flexible substrates.

These and other characteristics, features and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention. The reference figures quoted below refer to the attacheddrawings.

FIG. 1 shows a graph illustrating surface charge density D on acapacitor versus an applied electric field E. (ref.: M. E. Lines and A.M. Glass in ‘Principles and Applications of Ferroelectrics and RelatedMaterials)

FIG. 2 shows a non-volatile memory device according to the prior art.

FIGS. 3-4 and 6-7 illustrate subsequent steps in the processing of anon-volatile ferroelectric memory element according to an embodiment ofthe present invention.

FIG. 5 is a graph illustrating ferroelectric hysteresis loops beforecrosslinking and after crosslinking.

FIG. 8 shows hysteresis on a Id-Vg characteristic (or hysteretictransfer curve) of a blend of OC1OC10-PPV and PCBM based ferroelectrictransistor recorded on a device with a channel length/channel width=4/1000 μm.

FIG. 9 shows hysteresis on Id-Vg characteristics of a ferroelectrictransistor based on poly(3,9-di-tert-butylindeno[1,2-b]fluorene) (PIF).

In the different figures, the same reference figures refer to the sameor analogous elements.

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps. Where an indefinite or definite article is used when referring toa singular noun e.g. “a” or “an”, “the”, this includes a plural of thatnoun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

In FIGS. 3-4 and 6-7 subsequent steps in the processing of anon-volatile ferroelectric memory device according to an embodiment ofthe present invention are illustrated.

In a first step, a substrate 11 may optionally be planarized by forexample deposition of a planarization layer 12, which may for example bean epoxy- or novolac-based polymer, onto the substrate 11 (FIG. 3).

In embodiments of the present invention, the term “substrate” mayinclude any underlying material or materials that may be used, or uponwhich a device, a circuit or an epitaxial layer may be formed. In otheralternative embodiments, this “substrate” may include a semiconductorsubstrate such as e.g. a doped silicon, a gallium arsenide (GaAs), agallium arsenide phosphide (GaAsP), an indium phosphide (InP), agermanium (Ge), or a silicon germanium (SiGe) substrate. The “substrate”may include for example, an insulating layer such as a SiO₂ or an Si₃N₄layer in addition to a semiconductor substrate portion. Thus, the termsubstrate also includes silicon-on-glass, silicon-on sapphiresubstrates. The term “substrate” is thus used to define generally theelements for layers that underlie a layer or portions of interest. Also,the “substrate” may be any other base on which a layer is formed, forexample a glass, plastic or metal layer. The planarisation layer 12 maybe deposited onto the substrate 11 by means of for example spincoating.After optional planarisation of the substrate 11, a first conductivelayer is deposited onto the planarisation layer 12 or, in case thesubstrate 11 has not been planarised, onto the substrate 11 by anysuitable technique, for example by means of spincoating, drop casting,Doctor Blade, lamination of a prefabricated composite film, spraying orprinting. The first conductive layer may for example be a metal layer(e.g. gold, ITO), a conductive polymer layer (e.g. polyaniline dopedwith camphor sulfonic acid (PANI/CSA) orpoly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonaat)(PEDOT/PSS)) or any other suitable conductive material layer. Thethickness of the first conductive layer to be used depends on therequired sheet resistance for the application envisioned and thespecific resistance of the material that is used. The first conductivelayer may have a thickness of for example 100 nm and lower, e.g. in casethe first conductive layer is gold, the thickness of the firstconductive layer may for example 50 nm. On the other hand, if the firstconductive layer is PEDOT/PSS, the thickness may for example be 100 nm.

After deposition, the first conductive layer is patterned to form a gateelectrode 13. This may be done by for example standard photolithography.The photolithography process comprises the following subsequent steps.First, a photoresist layer is applied on top of the first conductivelayer, e.g. by means of spincoating. The photoresist layer may forexample have a thickness of a few μm an may be made of any suitablepolymer that can be used as a photoresist such as for example poly(vinylcinnamate) or novolak-based polymers. Thereafter, a mask is applied toalign a pattern onto the substrate 11. The photoresist layer is thenilluminated through the mask e.g. by means of UV light. Afterillumination, the photoresist is developed by which either theilluminated parts of the photoresist (positive resist) or thenon-illuminated part of the photoresist (negative resist) are removed,depending on which type of photoresist has been used. Patterning of thefirst conductive layer is then performed using the developed photoresistlayer as a mask, after which the remaining parts of the photoresistlayer are removed, typically by using an organic solvent. The result isshown in FIG. 3.

In case the first conductive layer is a conducting polymer layer,patterning can be done photolithographically, using the proceduredescribed by Touwslager et al. [Touwslager, F. J., Willard, N. P., & deLeeuw, D. M. I-line lithography of poly-(3,4-ethylenedioxythiophene)electrodes and application in all-polymer integrated circuits Appl.Phys. Lett. 81, 4556-4558 (2002)] and Gelinck et al. [G. H. Gelinck etal., Appl. Phys. Lett., 77, 1487 (2000).] Patterning can also be doneusing non-lithographic techniques known in the art, such as for instancesilk-screen printing, inkjet printing in case of soluble conductingpolymers, or for instance microcontact printing in case of gold, or forinstance microembossing in case of ITO.

In a subsequent fabrication step, which is illustrated in FIG. 4, anorganic ferroelectric layer 14 is deposited on top of the gate electrode13. The organic ferroelectric layer 14 may be applied by means of forexample spincoating from a solvent such as for example acetone,2-butanone, cyclohexane, dimethylsulfoxide (DMSO) or dimethylformamide(DMF). Furthermore, deposition of the organic ferroelectric layer 14 maybe performed by: drop casting, Doctor Blade, lamination of aprefabricated composite film, spraying or printing. The organicferroelectric layer 14 may have a thickness of for example 2000 nm orlower, preferably the organic ferroelectric layer 14 has a thicknesslower than 500 nm. The ferroelectric layer 14 may for example betriglycine sulphate (TGS) or may be a ferroelectric polymer or oligomerlayer based on random copolymers of vinylidenedifluoride (VDF) withtrifluoroethylene (TrFE) or with chlorotrifluoroethylene and otherfluorinated polymers, or more general the ferroelectric polymer oroligomer layer 14 may be a halogenated polymer. However, for theprocessing of memory devices, fluorinated polymers seem to have the mostbeneficial properties, because for memory applications it is importantthat the remnant polarization P_(r) of the ferroelectric polymer is ashigh as possible. Hence, materials having a high density of large dipolegroups are preferred such as is the case in fluorine containingpolymers, which have a remnant polarization >10 mC/m², for example ˜100mC/m². Another important reason for P_(r) not to be too low is that thestability of the stored of states (polarizations) will be at leastpartly dependent on it. In this respect also the coercive field isimportant. A too high E_(c) results in high switching voltages(generally 2×E_(c)×layer thickness for polarization saturation).However, a too low E_(c) may result in manifestation of detrimentalpolarization fields within the capacitors when connected to othercircuitry having parasitic capacitance. Furthermore, the thermal windowwherein the polymers have their ferroelectric effect is veryadvantageous for the fluorinated polymers in order to be used for memoryfunction. Thus, although other polymers or molecules exist, the fluorinecontaining materials seem to have the most beneficial properties, if thedevice formed is meant for memory applications.

The fluorinated polymer may preferably be a main chain polymer. However,the fluorinated polymer may also be a block copolymer or a side chainpolymer. The fluorinated polymer may for example be (CH₂—CF₂)_(n),(CHF—CF₂)_(n) (CF₂—CF₂)_(n) or combinations thereof to form (random)copolymers such as for example: (CH₂—CF₂)_(n)—(CHF—CF₂)_(m) or(CH₂—CF₂)_(n)—(CF₂—CF₂)_(m).

Other ferroelectric polymers may be used such as for example oddnumbered nylons, cyanopolymers (polyacrylonitriles, poly(vinylidenecyanide) and the polymers with a cyano group in the side chain),polyureas, polythioureas and polyurethanes. All polymers may be used inpure form or diluted within another (polymer) matrix.

Ferroelectric materials are discussed in “Principles and Applications ofFerroelectrics and related materials”, M. E. Lines and A. M. Glass,Oxford Press, 2001. A list of polymeric ferroelectric materials can befound in ‘Ferroelectric polymers, chemistry, physics and applications’,edited by Hari Singh Nalwa, Marcel Dekker, Inc 1995.

The organic ferroelectric layer 14 may then be patterned to form contactholes 15 to the first conductive layer where necessary (FIG. 4). Ifpossible, and this depends on the kind of material used for theferroelectric layer 14, the patterning may be carried out by means ofstandard photolithography as discussed above. However, in case theorganic ferroelectric layer 14 is based on fluorinated polymers,application of standard photolithography for patterning is difficult,because a fluorinated polymer dissolves in the polar organic solventscommonly used to remove the photoresist, which results in a completelift off of all layers on top. Therefore, in that case, the organicferroelectric layer 14 may yet be patterned by means of standardphotolithography by addition of a radiation crosslinking agent, such asa photosensitive crosslinking agent, which may for example be adiazocompound or a bisazide compound, to the fluorinated polymerspincoat solution. After spincoating of the ferroelectric polymer layer14 with the cross-linker, the ferroelectric layer 14 is irradiated withUV light through a mask which leads to a partially non-soluble layer.Instead of spin-coating other suitable application methods can be usedsuch as silk-screen printing or ink jet printing. Non-solubility of theorganic ferroelectric layer 14 is accomplished by means of crosslinkingof the polymer. The parts of the ferroelectric polymer layer 14 whichare not illuminated, and which thus do not cross-link, may besubsequently removed by washing with a suitable solvent, such as anorganic solvent, for example acetone, leaving a patterned film that maybe annealed at 130-145° C. to increase the ferroelectric properties ofthe layer 14. The annealing temperature depends on the exact polymercomposition. For example, in case of VDF/TrFE, the annealing temperaturedepends on the ratio of VDF and TrFE. Ferroelectric hysteresis loops maythen be measured with for example a Sawyer-Tower set-up at 10 Hzsinusoidal voltage. The ferroelectric hysteresis loops, beforecrosslinking (graph 1 in FIG. 5) and after crosslinking (graphs 2 and 3in FIG. 5) are compared in FIG. 5. In the latter case, hysteresis loopsboth with annealing (graph 2 in FIG. 5) and without annealing (graph 3in FIG. 5) are shown. From FIG. 5 it is clear that annealing almostdoubles the remnant polarization P_(r), which corresponds to the statethe memory cell resides in when the voltage of the power source isturned off. The crosslinking does not substantially alter theferroelectric switching behavior; while E_(c) is unaffected, P_(r)decreases slightly. However, it greatly improves stack integrity,because upon further processing the crosslinked organic ferroelectriclayer 14 will not dissolve. After patterning the organic ferroelectriclayer 14, a second conductive layer is deposited on top of the patternedorganic ferroelectric layer 14. The second conductive layer also fillsthe contact holes 15 formed in the organic ferroelectric layer 14, thusforming a vertical interconnect 16. This is illustrated in FIG. 6. Thesecond conductive layer may have the same thickness as the firstconductive layer. Again, the thickness of the second conductive layer tobe used depends on the required sheet resistance for the applicationenvisioned and the specific resistance of the material that is used. Thesecond conductive layer may for example be a metal layer (e.g. gold,ITO), a semiconducting layer, a conducting polymer layer (e.g. bepolyaniline doped with camphor sulfonic acid (PANI/CSA) orpoly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonaat)(PEDOT/PSS)), or may be any suitable conductive material layer. Thematerial of which the first and second conductive layers are formedshould be such that it is possible to construct low-ohmic verticalinterconnects 16.

Deposition of the second conductive layer may be done by means of anysuitable deposition technique, depending on the material that has beenused, such as for example chemical vapour deposition (CVD), spincoating,dropcasting, doctor blade, lamination of a prefabricated composite film,etc.

However, if the second conductive layer is a conductive polymer layerthat has to be spincoated onto the organic ferroelectric layer 14, theaqueous solution, from which the polymer layer is deposited, requiresmodification of the spincoating solution, because spincoating of thesecond conductive layer from a aqueous solution onto the organicferroelectric layer 14 results in severe dewetting. This may be overcomeby improving the wettability properties of the spincoating solutionthrough addition of a surface tension reducing agent, which may be anysolvent that is miscible with water, evaporates slower than water anddoes not attack the organic ferroelectric layer 14 within the timeperiod of the processing of the device. Examples of wetting agents whichmay be used in the present invention are for example n-butanol, an amineor a soap like reagent. Wetting agents comprising an amine preferablyhave a structure comprising an amine at one side and a polar group atthe other side. The polar group then makes the surface hydrophobic. Thepolar group may for example be an OH group. Specific examples of amineswhich may be used in the present invention may for example beamino-alcohols such as e.g. 6-amino-1-hexanol or 6-amino-1-dodecanol.Compounds with other polar groups may also be used, such as for examplea carboxylic acid as long as they do not easily dissociate. Soap likereagents which may be used in this invention are soaps comprising groupswhich form hydrogen bridges with the organic ferroelectric polymer 14,such as e.g. sulfoxides.

Patterning of the second conductive polymer layer may be carried out bymeans of for example standard photolithography as described earlier. Bypatterning the second conductive polymer layer a source 17 and a drain18 electrode are formed (FIG. 5).

In a further step, which is illustrated in FIG. 7, the memory device 30,formed by means of the present invention, is completed by depositing anorganic ambipolar semiconductor layer 19 on top of the source 17 and thedrain 18 electrode. The organic ambipolar semiconductor layer 19 may forexample comprise a layer in which p- and n-type organic semiconductorsare intimately mixed by which the drawback of switching polarization indepletion, which appears in the prior art devices, is circumvented,since now both polarization states of the ferroelectric gate can becompensated for by accumulation charges; one with holes accumulated inthe p-type part of the organic ambipolar semiconductor layer 19 and theother by accumulation of electrons within the n-type part of the organicambipolar semiconductor layer 19. Suitable organic ambipolarsemiconductor layers 19 that may be used in the present invention areblends of n- and p-type organic semiconductors such as for example amixture of [6,6]-phenyl C61 butyric acid methyl ester (PCMB) andpoly[2-methoxy, 5-(3,7)dimethyl-octyloxy]-p-phenylene vinylene(OC1OC10-PPV), a single polymeric semiconductor such as for examplepoly(3,9-di-tert-butylindeno[1,2-b]fluorene) (PIF)) or a doublelayer-stack of p-type and n-type semiconductors, wherein two activesemiconductors may be used.

The organic ambipolar semiconductor layer 19 may be patterned, but thisis step is not necessary. However, patterning of the organic ambipolarsemiconductor layer 19 may be performed in order to reduce leakagecurrents. Hence, whether or not patterning is performed depends on theapplication requirements.

FIG. 7 shows the memory device 30, processed according to the presentinvention. The device 30 comprises a transistor 20 and a via 21. Thetransistor 20 comprises a gate electrode 13 and source 17 and drain 18electrodes. An organic ferroelectric layer 14 is sandwiched in betweenthe gate electrode 13 and the source 17 and drain 18 electrodes.

The use of an ambipolar blend results in improved memory effects. Thismanifests itself in larger memory windows, larger current ratio betweenthe “0” and the “1” state, facilitating read-out operation. It also mayresult in faster switching times when device speed is limited by the RCtime of the semiconducting channel 30. In case of ambipolar devices,switching occurs in both polarities in accumulation. This makes itpossible to optimise on-current and off-current independently.

In FIG. 8 the Id-Vg characteristics of a blend of OC1OC10-PVV and PCMBbased ferroelectric transistor. The gate bias was swept from +40V to−40V and back at a constant scan speed of 1 V/s. The application of alarge voltage to the gate sets the direction of the polarization, andhence the value of the drain current of the transistor at VG=0. These,hysteric transfer curves may be repeated several times, e.g. 10 times,with only small degradation.

In a specific embodiment of the present invention a non-volatileferroelectric memory device 30 as described in the first embodiment isformed by using PEDOT/PSS for the first and second conductive layers andP(VDF/TrFE) is used for the organic ferroelectric layer 14. Themanufacturing process of the device of this specific embodiment isanalogous as described in the first embodiment of this invention. Thedevice 30 such formed may be combined with any suitable organicambipolar semiconductor layer 19. However, in a specific example of thisembodiment, the organic ambipolar semiconductor layer 19 may be a singlepolymeric semiconductor. In that way, an all-polymer non-volatile memorydevice may be processed using the method of the present invention.Hence, the method of the present invention may be used in themanufacturing of all-polymer devices.

In another specific embodiment a mixture of [6,6]-phenyl C61 butyricacid methyl ester (PCMB) and poly[2-methoxy, 5-(3,7)dimethyl-octyloxy]-p-phenylene vinylene (OC1OC10-PPV) may be applied asan organic ambipolar semiconductor layer 19. The above mentioned mixturemay be prepared by dissolving a mixture of PCMB and OC1OC10-PPV, with aratio of 4:1, in chlorobenzene. The weight content is about 0.5%. Thesolution was stirred for 1 hour at 80° C., cooled down to roomtemperature and then spincoated onto the patterned second conductivepolymer layer. The hysteresis loop of a ferroelectric transistor basedon a blend of PCMB and OC1OC10-PPV (channel length/channel width 4/1000μm) is shown in FIG. 8.

In yet another specific embodiment, an ambipolar transistor based on asingle organic semiconductor is provided. The organic ambipolarsemiconductor layer 19 used in this embodiment ispoly(3,9-di-tert-butylindeno[1,2-b]fluorene) (PIF). Id-Vg hysteresisloops of the PIF based transistor for different gate voltage ranges isshown in FIG. 9. The Id currents are low as a result of the low electronand hole mobility, nevertheless there is a clear evidence of the memoryeffect, and switching occurs at both polarities. The use of a singlematerial has, with respect to the use of a blend op n- and p-typematerial, the advantage that phase segregation of the p- and n-typecomponents cannot occur.

An advantage of the device 30 according to the first and secondembodiment of the present invention is that, since all layers may beprocessed from a solution, easy processing and a low cost technology isachieved. Including a mask to pattern the organic ambipolarsemiconductor layer 19, the whole process consists of only four masksets. Since the maximum processing temperature is below 150° C., thistechnology is compatible with the use of flexible substrates such as forexample polymer substrates.

A further advantage may be found in the fact that the dielectricconstant of ferroelectric organic materials, such as for exampleP(VDF/TrFE), is about three times larger compared to conventionalphotoresists which are used in the prior art. Hence, driving voltagesare reduced resulting in for instance lower power dissipation.

The improved memory effect and switching, described in the aboveembodiments, are independent of the material of the organicferroelectric layer 14 which is being used in combination with theorganic ambipolar semiconductor layer 19.

The memory window of the device 30 depends on the ratio of electroncurrent and hole current. For ratio's close to 0 or 1 the memory windowis largest. In the exceptional case that these currents are exactlysymmetrical with respect to Vg=0, then read out at Vg=0 is not possible,and read out of the memory state should be done at Vg≠0 and whereby Vgis smaller than the switching field.

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices according to the present invention, variouschanges or modifications in form and detail may be made withoutdeparting from the scope and spirit of this invention.

A non-volatile ferroelectric memory device is proposed which comprises acombination of an organic ferroelectric polymer with an organicambipolar semiconductor. The devices of the present invention arecompatible with—and fully exploit the benefits of—polymers, i.e.solution processing, low-cost, low temperature layer deposition andcompatibility with flexible substrates.

1. A non-volatile memory device comprising: an organic ambipolarsemiconductor layer in contact with at least two sides of a firstelectrode and a second electrode; and an organic ferroelectric layer incontact with one side of each of the first electrode and the secondelectrode and at least two sides of a control electrode, said organicambipolar semiconductor layer and said organic ferroelectric layer beingat least partially in contact with each other.
 2. The non-volatilememory device according to claim 1, wherein the control electrode isformed in a first conductive layer.
 3. The non-volatile memory deviceaccording to claim 2, the control electrode being separated from saidorganic ambipolar semiconductor layer by said organic ferroelectriclayer.
 4. The non-volatile memory device according to claim 2, whereinthe first electrode and the second electrode are formed in a secondconductive layer, said first and said second electrodes being separatedfrom each other by material of the organic ambipolar semiconductorlayer, and said first and said second electrodes being separated fromsaid control electrode by said organic ferroelectric layer.
 5. Thenon-volatile memory device according to claim 4, wherein the secondconductive layer is a conductive polymer layer.
 6. The non-volatilememory device according to claim 5, wherein the conductive polymer layeris a PEDOT/PSS layer or a PANI layer.
 7. The non-volatile memory deviceaccording to claim 2, wherein the first conductive layer is a conductivepolymer layer.
 8. The non-volatile memory device according to claim 7,wherein the conductive polymer layer is a PEDOT/PSS layer or a PANIlayer.
 9. The non-volatile memory device according to claim 1, whereinthe organic ferroelectric layer is a ferroelectric polymer or oligomerlayer.
 10. The non-volatile memory device according to claim 9, whereinthe ferroelectric polymer or oligomer layer is a layer comprisingmaterial selected from: (CH2-CF2)_(n), (CHF—CF2)_(n)(CF2-CF2)_(n) orcombinations thereof to form (random) copolymers including(CH2-CF2)_(n)—(CHF—CF2)_(m) or (CH2-CF2)_(n)—(CF2-CF2)_(m).
 11. Thenon-volatile memory device according to claim 1, wherein the organicambipolar semiconductor layer comprises a mixture of an n-type and ap-type semiconductor material.
 12. The non-volatile memory deviceaccording to claim 11, wherein the organic ambipolar semiconductor layercomprises a mixture of [6,6]-phenyl C61 butyric acid methyl ester andpoly[2-methoxy,5-(3,7)dimethyl-octyloxy]-p-phenylene vinylene.
 13. Thenon-volatile memory device according to claim 1, wherein the organicambipolar semiconductor layer comprises a single organic material. 14.The non-volatile memory device according to claim 13, wherein the singleorganic material is poly(3,9-di-tert-butylindeno[1,2-b]fluorene). 15.The non-volatile memory device according to claim 1, the memory devicecomprising a memory window, whereby said memory window depends on aratio of electron current and hole current.
 16. The non-volatile memorydevice according to claim 15, whereby said ratio of electron current andhole current is close to 0 or close to
 1. 17. A method for processing anon-volatile memory device, the method comprising acts of: forming anorganic ferroelectric layer in contact with at least two sides of afirst electrode and a second electrode; and forming an organic ambipolarsemiconductor layer in contact with one side of each of the firstelectrode and the second electrode and at least two sides of a controlelectrode, said organic ambipolar semiconductor layer and said organicferroelectric layer being at least partially in contact with each other.18. A non-volatile memory device comprising: first and secondelectrodes; a control electrode; an organic ambipolar semiconductorlayer in contact with three sides of said first and second electrodes;and an organic ferroelectric layer in contact with one side of each saidfirst and second electrodes, at least two sides of said controlelectrode and at least partially in contact with said organic ambipolarsemiconductor layer.
 19. The device of claim 18, further comprising aplanarization layer in contact with one side of said control electrode.